1. Field of the Invention
The present invention concerns the production of rims with spokes for vehicle wheels by means of a single casting.
2. History of the Related Art
Wheel rims are known, which consist of several parts made of stamped and/or forged sheet material.
Such rims are rather heavy and unpleasing to look at; they are normally used for mass-produced economical cars and are provided with covers.
Wheel rims are also known, which are obtained from the casting of metal in a single piece. Such rims, the spokes of which have a U-shaped section, are characterized by a pleasant look and are mounted on medium-high class cars. They are normally made of metal alloys with low specific weight.
It is well known that, with the same material, a hollow tubular element is much more resistant than a solid one.
Any kind of wheel rim does not only provide for connection between a tire and the vehicle, but has also other functions.
The air inside the tire heats while the vehicle is moving, thus heating also the metal rim; the air outside the tire touches the metal surfaces and thus reduces the overheating of the air inside the tire and therefore of the whole wheel.
The larger the exchange surfaces in the tire air-metal rim-environment, the better is the cooling of the whole wheel.
When the wheels are used on uneven ground or when there are irregularities in the road layout, part of the stress is absorbed by the tire, which is subject to elastic deformation, thus causing the compression of the air inside the tire. Therefore, the greater is the air volume inside the tire, the lesser is the stress transmitted to the vehicle by the wheel. The above-mentioned problems are further increased in races.
At present more castings are made to obtain alloy rims with hollow spokes; a first casting is made to obtain the hub and the hollow spokes and a second casting to obtain the whole rim, by putting the hub and the already cast spokes inside the die.
The need to make two successive castings doesn't make it possible to get very good results, since the obtained rim doesn't consist of homogeneous material and has less resistance along the surface joining the two successive castings.
To carry out the rim with the above-described system sand cores are used which, during the casting, can partly mix with the metal and modify its characteristics, leave the surfaces irregular and rough and can cause microcracks. Such microcracks are invisible to the naked eye, but allow air emissions from the inside of the tire, thus making the rim unusable for tubeless tire.